Equivalent time sampling is a process that allows a repetitive, high-speed signal to be sampled and held at a lower sample rate. For example, in an Equivalent Time (ET) sampling radar system, a Radio Frequency (RF) pulse is transmitted in a repetitive fashion. For each repetition of the transmit pulse, a window of the received signal is sampled. The sample window is moved in time so as to sample a later portion of the received waveform for each repetition. This constantly increasing delay between a transmit pulse and its sample window generally corresponds to increasing distance from the transmitter, or in the case of ground penetrating radar, increasing depth in the soil.
FIG. 1 is an illustration of wave 100 undergoing equivalent time sampling. The positive edge of transmit trigger pulse 101 causes a transmitter (not shown) to transmit a signal that returns to the receiver as wave 100. The negative edge of receive trigger pulse 102 causes a receive unit (not shown) to listen to wave 100, thereby sampling it and holding it for a very short time. Each subsequent negative edge of receive trigger pulse 102 is increasingly offset from its corresponding transmit trigger pulse 101 positive edge so that subsequent samples advance over the contour of a cycle of wave 100. Sample and hold output 103 is a reconstructed pulse of wave 100, stretched out in time.
In one example, the repetition rate of the transmit and receive triggers is about sixteen Megahertz (MHz), such that sixteen million samples are taken per second. If the sample and hold circuit has an aperture window of about ten picoseconds (ps), and if the delay between a transmit trigger edge and its corresponding receive trigger edge is incremented by ten picoseconds per repetition, then the real sample rate of sixteen Megahertz has an equivalent sample rate of one hundred gigahertz (GHz). However, the effective bandwidth may be limited to about ten gigahertz due to inaccuracies in the aperture window and edge inaccuracies (jitter) in transmit and receive trigger pulses 101 and 102.
Conventional ET sampling systems can be used with time domain radar to effect a time-stretch of the received radar signals, as shown in FIG. 1. For example, if each pulse cycle of wave 100 lasts ten nanoseconds, it may be down-converted by an equivalent time sampling approach and stretched to an equivalent shape in a ten millisecond period.
With ET sampling systems, when the pulse repetition rate is constant, the system often undersamples external energy sources. This external radiation is received as coherently sampled and down converted. As a result, prior art radar systems tend to have increased susceptibility to any frequency that shows up as any harmonic of the sample rate. Thus, the above-described radar system will generally be expected to have increased susceptibility to interference for any external energy that shows up as any multiple of sixteen Megahertz when it is sampled and down converted. FIG. 2 is a graph showing susceptibility versus frequency for such a system. The width of the spikes in FIG. 2 is usually related to the equivalent time bandwidth of the system. Thus, if the equivalent time bandwidth of the system is one Megahertz, the width of the spike will be two Megahertz due to mirroring around the Nyquist frequency.
Increased susceptibility is often a problem for radar systems, because designers of such systems usually design based at least in part on the “weakest link.” Thus, relatively low susceptibility for some frequencies is usually irrelevant if there are large susceptibility spikes in other frequencies. One way that radar system designers mitigate the effects of increased susceptibility is to increase transmitter power so that more distant interference sources appear much weaker than the transmitted signal. However, this increases radiated emissions of the radar system.
Another kind of interference that is often seen by constant pulse repetition rate systems is interference from correlating frequencies-frequencies that are relatively close to the pulse repetition rate. A useful analogy to understand correlating frequencies involves the wheels of a car as seen on a movie screen. Often, the wheels of a car as seen on a movie screen appear to rotate slowly backward or forward. This is due to the relative rate of the wheels when compared to the rate of frame advance of the movie camera. If the wheels are rotating slightly slower that the rate of frame advance, the wheels will appear to rotate slowly backwards. Similarly, if the wheels are rotating slightly more quickly than the rate of frame advance, then the wheels will appear to rotate slowly forward. The same phenomenon occurs in ET sampling systems. An external fixed frequency that is close to the pulse repetition rate will be under sampled and aliased and will be down-converted to a coherent wave that interferes with the detection of the intended returned wave.
Yet another source of interference involved distant pulses from the radar system. Radar systems typically transmit a pulse and then turn on a receiver for a certain period of time in order to “listen” for any reflections occurring in that time range from nearby objects. However, in that same observation period, the radar system can also pick up reflections off of more distant objects for the prior transmit pulse, the transmit pulse preceding the prior pulse, etc. The more distant pulses are often interpreted as clutter. Constant sampling of such signals tends to make those signals appear coherent, such that they can cause a significant amount of interference.
Currently, there is no system available that minimizes interference from these and other sources without increasing radiated emissions or by significantly increasing the cost of the system (e.g., by using complex filtering techniques).